The objectives of the proposed research are to define the genetic structure of natural populations of the human-pathogenic bacterium Haemophilus influenzae on a global scale through analysis of multilocus chromosomal genotypes and to examine possible correlations between recently discovered patterns of geographic and temporal variation in clonal composition and diversity of the bacterial populations and patterns of racial/ethnic differentiation and historical demographic movement of the host human populations. Multilocus bacterial genotypes, clones, are defined on the basis of the electrophoretic mobility of allelic variants (electromorphs) of 16 metabolic enzymes and the electrophoretic pattern of the major outer-membrane proteins. The proposed research is the first attempt to assess on a worldwide scale the genetic diversity and clonal structure of a bacterial species and to examine the relevance of this structure to problems of pathogenicity, disease specificity, epidemiology, and vaccination strategies. Specific questions to be addressed are as follows: (1) why are the serotype beta populations in the United States different in clonal composition and much more diverse genetically than those in northwestern Europe and New Guinea? (2) Is there a causal relationship between degree of racial/ethnic mixing of human populations and degree of diversity in clonal composition of bacterial populations? (3) What is the extent of statistical association of the phylogenetically diverse types of H. influenzae with various diseases and anatomical sites of infection? (4) To what extent is the higher incidence of certain types of disease in some racial/ethnic groups, such as Alaskan Eskimos and American Indians, associated with distinctive population structures of the pathogen? (5) Does vaccination against serotype b H. influenzae affect the genotypic population structure of the pathogen? (6) What are the genetic and evolutionary relationships of the various capsular types and unencapsulated strains? (7) Do cell lines that have recently lost the capsule have any special clinical significance? The research will involve collaborative work with Dr. D. M. Granoff on outer-membrane protein patterns and Dr. E. R. Moxon on the structure of genes responsible for capsule production and expression.